Studies conducted in hospitals in the United States and Canada indicate that about 1.5 to 7.5 kg of biomedical waste is generated per bed per day (Ontario Ministry of the Environment "Biomedical Waste Incinerators" Incinerator Design and Operating Criteria Vol. II, October, 1986). Biomedical waste includes used syringes, gowns, bedding, containers, bandages, dressings, used disposable gloves, human waste and other liquid and solid waste materials which may be contaminated with, for example, infectious bacteria and viruses.
Incineration is presently a method of treating the biomedical waste. There are, however, concerns about the environmental impact of incineration, especially with respect to airborne emissions from incineration plants. Accordingly, incinerators must now be equipped with sophisticated emission quality control devices. Such devices are expensive, thereby providing a financial obstacle to the upgrading and/or building of incinerators.
Alternatives to incineration include autoclaves, chemical treatment, microwave and macrowave technologies. The most common alternative, is sterilization by steam in an autoclave. The process uses hot steam under pressure to kill bacteria, viruses, parasites and heat-resistant spores and is used extensively in a non-waste treatment manner in laboratories to sterilize equipment, media for bacterial growth and pathogenic cultures.
Sterilization of biomedical waste is achieved by exposing all portions of the waste to a temperature and pressure for a time sufficient to kill bacteria, viruses, parasites and heat-resistant spores. However, since biomedical waste is segregated and packaged in leak-proof, color designed plastic bags (red is the designated color in the United States and yellow is the designated colour in Canada) contained in sealed boxes, heat transfer must often be effected through tightly wrapped packages and plastic bags containing the waste material. The sterilization cycle must then be extended to ensure that all portions of the waste material have been subjected to the desired conditions of temperature and pressure for the appropriate length of time. Accordingly, the time required to achieve sterilization depends on the efficiency of heat transfer which in turn depends on the type of material, density of the material, batch volume and how full the autoclave is loaded. Heat transfer is even further inhibited by entrained air inside the package resulting in cold spots which can interfere with sterilization unless the cycle is sufficiently extended to ensure complete sterility.
Another difficulty is the inability to control internal pressure of the sealed bags and boxes. In particular, bags and packages can explode during the process inside the vessel, making unloading very messy despite the elimination of infectious hazards. The degree to which the contents of the autoclave will explode is somewhat dependent on the length of the cooling cycle after the desired sterilization cycle. This cooling cycle can extend the time in the autoclave by 100% or more.
Sterilization in an autoclave relies on injection of steam directly into the autoclave. Injected steam condenses on the walls of the autoclave and on the outer surfaces of the waste and containers thereof or is absorbed by the waste. The steam condensate is then drained from the autoclave for subsequent disposal. It will be appreciated by those skilled in the art that the steam condensate is generally unsuitable for reuse and represents a significant energy loss as the hot water is drained. Furthermore, the moisture absorbed by the waste can substantially increase the weight of the packages, for example, by about 50%. Accordingly, the moist packages are heavier, more difficult to handle and make unloading cumbersome. Moreover, since dumping costs at landfill sites are typically set on a per ton basis, the increased weight due to moisture retention represents substantial increases in dumping costs.
An alternative to conventional autoclaves is a process for the disposal of medical waste in a pressure vessel fitted with high-speed blades. The blades are provided at he base of the vessel and operate at high rotational speeds (900-3500 rpm). An internal mixer is provided on the lid to direct the waste towards the blades. Steam is injected directly into the vessel for heat transfer. At the end of the sterilization process, the vessel is vented to vacuum to flash off moisture.
Another alternative to the conventional autoclave, is a cylindrical pressure vessel with an elongated cylindrical drum located in the pressure vessel for receiving the waste to be treated. The pressure vessel and the drum are set at an angle, such that the end where the drum is open and the door of the pressure vessel is located is elevated relative to the other end. The drum has a series of lifting paddles on the wall for agitation of the waste material in the drum as the drum is rotated within the pressure vessel. In addition, the drum has a helical flight which work in a counter-current manner with the lifting paddles to mix the waste and when the drum is rotated in the other direction moves the waste out of the drum for removal through the door of the pressure vessel. Water is added to the waste to attempt to receive a content of 75% moisture in the waste. Steam is injected directly into the pressure vessel for heat transfer.